1. Field of the Invention
The present invention relates to the measurement of electrostatic field strength, and more particularly, to a fiber-optic sensor incorporating a conductive Fabry-Perot microcavity bounded by a thin conductive diaphragm for measuring the strength of electric and electromagnetic fields.
2. Description of the Background
The monitoring of electric fields in high voltage high-power equipment is of fundamental importance in the power industry. Both AC and DC fields must be measured in many high-voltage components in order to understand their operation, and to prevent or repair malfunctions. The need for monitoring has become especially apparent in recent months as a result of the public concern over radiation from high-power transmission lines.
Consequently, there exists a considerable demand for an unobtrusive, accurate and inexpensive electric field sensor which is dielectric or otherwise immune to electromagnetic interference.
Recent efforts to fulfil the demand have turned to fiber optic technology for its superior sensitivity, wide dynamic range, and immunity to electromagnetic interference.
For example, U.S. Pat. No. 4,933,629 issued to Kozuka, et al., discloses an optical fiber sensing device for measuring AC magnetic quantities based on the Faraday effect. Light is passed through a Faraday cell which phase-modulates the light in proportion to a surrounding magnetic field. The strength of the AC field can be calculated from the amount of phase modulation.
Similar interferometric electric field sensors have been proposed, for example, U.S. Pat. No. 4,899,042 issued to Falk, et al., U.S. Pat. No. 4,477,723 issued to Carome, et al., U.S. Pat. No. 4,631,402 issued to Nagatsuma, et al., and U.S. Statutory Invention Registration No. H371 issued to Bobb, all of which generally employ phase modulation to measure electric field strengths. However, the measurements carried out by the above-described devices are based on electro-optic phenomena such as the Pockel's and piezoelectric effects. Sensors based on electro-optic phenomena are unusually bulky and are highly prone to ambient temperature and pressure variations. It would be more advantageous to provide a fiber-optic sensor based on electrostatic force effects. This would eliminate the susceptibility to temperature and pressure variations. However, the electrostatic forces which result from electric fields are very weak and difficult to measure with accuracy. Currently available field-sensitive elements lack the necessary sensitivity, and they exhibit a non-linear response to varying fields. The non-linearity prevents an accurate relative measurement. The problem becomes especially severe for weak electric fields.
The problem of non-linearity arose previously in a different context. Researchers first began to exploit the mechanical properties of silicon during the integrated circuits revolution by developing microsensors to measure pressure. The development efforts led to a variety of thin film diaphragms which converted pressure to a proportionate electrical signal. For example, A. D. Kurtz, et al. (U.S. Pat. No. 3,654,579) disclose a piezoresistive pressure transducer employing a thin film diaphragm which is displaced by pressure. The displacement alters the resistivity of the transducer, and the change in resistivity is measured electronically. The thin film diaphragm improved the linearity of response to changing pressure.
Near perfect linearity was later achieved through the use of convoluted diaphragms, such as that disclosed in U.S. Pat. No. 4,467,656 issued to Mallon, et al. As shown in FIG. 1, the Mallon, et al. device is a pressure-sensing diaphragm formed with a plurality of concentric recesses or corrugations. As the piezoresistive diaphragm is displaced, the corresponding change in resistance can be measured at terminals 50 and 51. The corrugations significantly improve the linearity of response to pressure. The improved response allows a more accurate relative determination of pressure. Moreover, the corrugations can easily be produced on thin silicon diaphragms by conventional etching and diffusing techniques.
It would be greatly advantageous to employ corrugated diaphragm technology in developing an electrostatic force-effect sensor capable of linear modulation of light in a Fabry-Perot cavity connected to an optical waveguide, to improve thereby the accuracy of electric field measurement while reducing ambient pressure and temperature effects.